This invention relates to sulphonated polyimides, which find application particularly in the preparation of ion exchange membranes notably for the manufacture of fuel cells.
The use of solid polymer electrolytes was proposed in the 1950s and applied notably in the construction of fuel cells which were intended particularly to supply space craft with energy.
The interest in fuel cells is now progressing beyond the generation of power for space craft and the automobile industry has interest in them for at least two reasons.
the first rests on the concern to avoid pollution caused by internal combustion engines. In effect it is clear that it will be difficult to prevent all discharges of nitrogen oxides, unburnt hydrocarbons and oxygenated compounds by means of all the improvements that one can expect through better control of combustion.
the second reason, for the longer term, is to research motors that use a fuel other than the fossil fuels that it is known will not last for ever.
Any system based on hydrogen can respond to the concerns mentioned above. The source of supply is potentially inexhaustible and electrochemical combustion only produces water.
The schematic assembly of a fuel cell that permits at the same time the production of electrical energy and incidentally the synthesis of water for the needs of the crew of a space vehicle, is represented in part in FIG. 1 appended.
The ion exchange type of membrane formed from a solid polymer electrolyte (1), is used to separate the anode compartment (2) where oxidation occurs of the fuel, such as hydrogen H2 (4) according to the equation:
2H2xe2x86x924H++4exe2x88x92; 
from the cathode compartment (3) where the oxidant such as oxygen O2 is reduced according to the equation:
O2+4H++4exe2x88x92xe2x86x922H2O 
with production of water (6) while the anode and the cathode are connected through an external circuit (10).
The anode (7) and the cathode (8) are essentially constituted by a porous support, for example made of carbon, on which particles of a noble metal such as platinum are deposited.
The membrane and electrode assembly is a very thin assembly with a thickness of the order of a millimetre and each electrode is supplied from the rear with the gases using a fluted plate.
One very important point is to properly maintain the membrane in an optimum moisturised state so as to ensure maximum conductivity.
The membrane has a double role. On the one hand it acts as an ionic polymer permitting the transfer (9) of hydrated protons H3O+ from the anode to the cathode, and on the other hand it keeps each of the gases oxygen and hydrogen in their compartments.
The polymer constituting the membrane must therefore fulfil a certain number of conditions relating to its mechanical, physico-chemical and electrical properties.
First of all, the polymer must be able to give thin films, between 50 and 100 micrometers thick, which are dense and without defects. The mechanical properties, rupture stress modulus, ductility, must make it compatible with the assembly operations which include, for example, being clamped between metal frames.
The properties must be conserved when it passes from a dry to a moist state.
The polymer must have good thermal stability to hydrolysis and exhibit good resistance to reduction and oxidation up to 100xc2x0 C. This stability shows itself in terms of variation in ionic resistance and in terms of variation in mechanical properties.
Finally, the polymer must have high ionic conductivity, this conductivity is provided by strongly acidic groups such as phosphoric acid groups, but above all by sulphonic groups linked to the polymer chain. Because of this these polymers will generally be specified by their equivalent mass, that is to say, the weight of polymer in grams per acid equivalent.
By way of example, the best systems developed at present are capable of supplying a specific power of 1 W.cmxe2x88x922, or a current density of 4 A.cmxe2x88x922 for 0.5 Volts.
Since 1950, numerous families of polymers or sulphonated polycondensates have been tested as membranes without it being at present possible to establish with certainty the relationships between chemical structure, film morphology and performance.
At first, sulphonated phenolic type resins prepared by sulphonation of polycondensed products such as phenol-formaldehyde resins were used.
The membranes prepared with these products are low cost, but they do not have sufficient stability to hydrogen at 50-60xc2x0 C. for applications of long duration.
Next one turned towards sulphonated polystyrene derivatives which have greater stability compared with that of the sulphonated phenolic resins but cannot be used at more than 50-60xc2x0 C.
At the present time, the best results are obtained with copolymers, the linear main chain of which is perfluorinated and the side chain of which carries a sulphonic acid group.
These copolymers are commercially available under the trademark NAFION(copyright) from the Du Pont Company or ACIPLEX-S(copyright) from the Asahi Chemical Company. Others are experimental, products by the DOW Company for the manufacture of the membrane named xe2x80x9cXUSxe2x80x9d.
These products have been the subject of numerous developments and conserve their properties for several thousands of hours between 80 and 100xc2x0 C. with current densities that depend on the partial pressures of the gases and the temperature. The current density is typically 1 A.cmxe2x88x922 at 0.7 Volts for Nafion(copyright) 112 with a thickness of 50 xcexcm.
The polymers of the Nafion(copyright) type are obtained by co-polymerisation of two fluorinated monomers, one of which carries the SO3H group. A second route for obtaining perfluorinated membranes has been explored in documents by G.G. Scherer: Chimia, 48 (1994), p. 127-137; and by T. Monose et al., patent U.S. Pat. No. 4,605,685. It involves the grafting of styrene or fluorinated styrene monomers onto fluorinated polymers which are subsequently sulphonated. These membranes however have properties close to those of fluorinated co-polymers.
If one tries to draw lessons from the teachings of the prior art, it is apparent that the best chemical structure for a polymer that can be used in the form of a membrane for the exchange of protons corresponds to the following criteria:
a main chain totally perfluorinated
branches bearing a sulphonic acid group
equivalent weight between 800 and 1200.
In the documents by W. Grot ; Chem. Ing. Tech., 50, 299 (1978) and by G.G. Scherer : Phys. Chem., 94, 1008-1024 (1990) they claim for these structures xe2x80x9cvery good thermal stabilitiesxe2x80x9d; however, it should be taken into consideration that the notion of thermal stability has to be taken here as the ability to resist acid hydrolysis at a temperature between 60 and 100xc2x0 C. over a period of several thousands of hours and that therefore the information from these documents must be considered prudently.
To that, it would be proper to add resistance to oxidation in contact with oxygen in the cathode compartment and resistance to reduction in the presence of H2.
On the other hand, from the viewpoint of the development of fuel cells that can be used for automobile traction, another important problem that will henceforth be clearly identified by the experts is the cost of the membrane.
In 1995, the cost of membranes produced or under development was of the order of 3000 to 3500 French francs per square meter and one might estimate that it would be necessary to divide this cost by 10 or indeed 20 in order for it to play a part in the industrial development of fuel cells for the automobile industry.
With a view to lowering the costs, poly 1,4-(diphenyl-2,6)-phenyl ethers, sulphonated on the main chain, the polyether-sulphones and polyether-ketones have been synthesised and tested without really holding their own against the fluorinated membranes with regard to their immediate performance and their durability.
In effect, the rigidity of the chains makes these products insoluble and it becomes difficult to obtain the thin films necessary for the creation of the membranes.
There therefore exists an unsatisfied need for polymers which can be easily made into the form of membranes, namely of thin films which meet the conditions already mentioned above relating notably to their mechanical, physico-chemical and electrical properties, in particular those relating to their thermal stability and their resistance to acid hydrolysis, at elevated temperature for a long period of time, their resistance to oxidation in contact with oxygen as well as their resistance to reduction in the presence of hydrogen.
Furthermore, there exists a need for membranes which, at the same time as satisfying the properties above, can be manufactured at low cost, by a simple method with raw materials that are readily available.
The aim of this invention is to provide a polymer which satisfies the group of needs previously mentioned.
A further aim of the invention is to provide membranes that include or are prepared with this polymer and a fuel cell that includes these membranes.
These aims and others are met conforming to the invention by a polyimide that comprises repeating structures of formula (In). 
and repeating structures of formula (Im) 
in which
the groups C1 and C2 can be identical or different and each represent a tetravalent group that includes at least one aromatic carbon ring possibly substituted and having from 6 to 10 atoms of carbon and/or a heterocyclic ring with aromatic character, possibly substituted and having from 5 to 10 atoms and including one or more heteroatoms chosen from among S, N, and O; C1 and C2 each forming with the neighbouring imide groups rings with 5 or 6 atoms.
The Ar1 and Ar2 groups can be identical or different and each represent a divalent group that includes at least one aromatic carbon ring possibly substituted and having from 6 to 10 atoms of carbon and/or a heterocyclic ring with aromatic character, possibly substituted and having from 5 to 10 atoms and including one or more heteroatoms chosen from among S, N and O; at least one of said aromatic carbon rings and/or heterocyclic rings of Ar2 being, in addition substituted by at least one sulphonic acid group.
The repeating structure (In) is repeated k times and the repeating structure (Im) is repeated k times, j and k being two whole numbers.
Preferably, j represents a whole number from 1 to 200, more preferably from 4 to 60 and k represents a whole number from 1 to 300, preferably from 4 to 120.
The co-polymer according to the invention, depending on the positioning of the two structures which make it up, can be defined as being a sequential, alternating or a statistical co-polymer.
However the polyimide according to the invention which can be defined as a sulphonated polyimide corresponds preferably to the following general formula (I): 
in which C1, C2, Ar1 and Ar2 have the meanings already given to them above and where each of the groups R1 and R2 represent NH2 or a group of formula 
where C3 is a divalent group that includes at least one aromatic carbon ring possibly substituted and having from 6 to 10 carbon atoms and/or a heterocyclic ring with aromatic character, possibly substituted and having from 5 to 10 atoms and including one or more heteroatoms chosen from among S, N and O.
C3 forming with the neighbouring imide group a ring with 5 or 6 atoms.
In the formula (I) above:
m represents a whole number preferably from 1 to 20, more preferably from 2 to 10;
n represents a whole number preferably from 1 to 30, more preferably from 2 to 20;
o represents a whole number preferably from 1 to 10, more preferably from 2 to 6;
The molecular weight of the polyimide according to the invention is generally from 10,000 to 100,000, preferably from 20,000 to 80,000.
The equivalent molecular weight of the polyimide according to the invention is preferably from 400 to 2500, more preferably from 500 to 1200.
Because of this, the numbers m and n (j and k) will be chosen in such a way that the equivalent molecular weight shall be from 400 to 2500, preferably from 500 to 1200, the equivalent molecular weight having been defined above.
In a general way, it is known that the heterocyclic polymers and in particular the polyimides can allow one to obtain films thanks to their synthesis in two steps.
These xe2x80x9cheterocyclicxe2x80x9d polymers are used, for example in aeronautical and space applications which require excellent mechanical properties and good resistance to oxidation. These applications are very much removed from the field of this patent application.
The specific sulphonic co-polyimides of this invention offer, in a surprising manner, all the properties already mentioned above as being required for the production of membranes, in particular cation exchange membranes, notably for fuel cells, the performances of which are compatible with the envisaged applications.
In particular, The specific co-polymers according to the invention can be easily formed into films or membranes of a suitable thickness.
The polymers according to the invention have a very high ion exchange capacity greater than 0.4 meq/g, for example from 0.8 to 2.5 meq/g, which is greater than the ion exchange capacity of the polymers of the prior art which generally achieve only a maximum of 0.9 to 1.2 meq/g.
The membranes comprising the polymers according to the invention also have high thermal stability, for example to acid hydrolysis at high temperature, that is to say for the most stable membranes up to temperatures that can reach, for example 100xc2x0 C., and this for a long duration that can extend, for example, to 5000 hours.
These conditions are the conditions of use that can prevail in the cells where the membranes are put to use.
Similarly, the membranes according to the invention have excellent resistance to reduction and to oxidation.
The invention is therefore totally dissociated from the prior art mentioned above in which the polymers recommended for the manufacture of membranes for the exchange of cations and in particular protons, notably for fuel cells have a structure fundamentally different from that of the polymers of the polyimide type of the present application for a patent.
This patent application departs radically and in a surprising way from the processes of the prior art by preparing specific polyimides with a view to their use in cation exchange membranes.
In effect, in a general way, the polyimides have been neither mentioned not proposed for use in this field, on the other hand, the specific polyimides according to the invention have mechanical, physico-chemical and electrical properties superior to those of the polymers of the prior art as is demonstrated below.
Nothing allowed one to suppose that the polyimides according to the invention were going to totally satisfy the requirements expressed and until now not satisfied for the preparation of cation exchange membranes.
Finally, as described below, the polyimides according to the invention are prepared in a simple manner, by methods proven on the industrial scale and from raw materials that are available and which are low cost. Because of this, the membranes obtained and the fuel cells that include these membranes will also see their costs much reduced.